The present invention relates generally to reciprocating piston internal combustion engines. More particularly, the invention relates to a system and method for varying the compression ratio of a reciprocating internal combustion engine.
The xe2x80x9ccompression ratioxe2x80x9d of an internal combustion engine is defined as the ratio of the volume in a cylinder above a piston when the piston is at bottom-dead-center (BDC) to the volume in the cylinder above the piston when the piston is at top-dead-center (TDC). The higher the compression ratio, the more the air and fuel molecules are mixed and compressed and the better the efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the internal combustion engine.
In conventional internal combustion engines, however, the compression ratio is fixed and thus the engine efficiency cannot be changed to yield optimal performance. Accordingly, so-called xe2x80x9cvariable compression ratioxe2x80x9d (VCR) internal combustion engines have been developed to vary the clearance volume of a cylinder in order to achieve improved fuel economy and increase engine power performance. Such VCR engines are designed to have a higher compression ratio during low load conditions, and a lower compression ratio during high load conditions. Conventional techniques include using xe2x80x9csub-chambersxe2x80x9d and xe2x80x9csub-pistonsxe2x80x9d to vary the volume of the cylinder, see for example U.S. Pat. Nos. 4,246,873 and 4,286,552; varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, see U.S. Pat. No. 5,865,092; and varying the actual length of the connecting rod itself, see U.S. Pat. Nos. 5,724,863 and 5,146,879.
Other techniques include the use of eccentric rings or bushings either at the lower xe2x80x9clargexe2x80x9d end of a connecting rod or the upper xe2x80x9csmallxe2x80x9d end of the connecting rod for varying the length of the connecting rod or height of the reciprocating piston. In U.S. Pat. Nos. 5,562,068, for example, a variable compression ratio connecting rod is disclosed wherein the effective length of the rod is varied via an eccentric ring that can be rotated and selectively locked to the crank pin and to the large end of the rod. When a working hydraulic pressure is released, the eccentric ring becomes locked with the crank pin thus resulting in a longer effective rod length and therefore a higher compression ratio mode. When the working hydraulic pressure is applied, the eccentric ring becomes locked with the rod, thus resulting in a shorter effective rod length and therefore a lower compression ratio. Transition from the higher compression ratio mode to the low compression ratio mode, or vice-versa, occurs when the piston is at BDC.
U.S. Pat. No. 5,960,750 similarly discloses a connecting rod having a rotatable eccentric ring in communication with a connecting rod. The connecting rod includes a mechanically actuated locking member for locking the eccentric ring in one of two positions. When actuated in a first direction, the locking member locks the eccentric ring in a position corresponding to a maximum effective length of the connecting rod, i.e., a high compression ratio mode. When actuated in a second direction, the locking member locks the eccentric ring in a position corresponding to a minimum effective length of the connecting rod, i.e., a high compression ratio mode.
U.S. Pat. No. 5,417,185 and Japanese Publication JP-03092552 also disclose eccentric rings, each of the rings however being disposed between the xe2x80x9csmallxe2x80x9d end of a connecting rod and a corresponding piston pin. The rings are used to vary piston height and thus compression ratio.
The eccentric ring devices of the above cited references however are undesirable in that each eccentric ring must be rotated at least 180 degrees before one of the desired operating modes or positions is engaged. Locking of the ring in a proper position may not occur within an optimum period of time, for example during a single engine cycle, thus leaving the effective length of the rod and consequently the compression ratio of the cylinder in an undesired intermediate state. In an alternative embodiment, the ""750 device in fact requires a gear pump to assist rotation of the eccentric bushing. The ""068 device in addition requires the piston to be at BDC for a transition to occur from high to low compression mode.
Accordingly, the inventors herein have recognized the need to provide an improved variable compression ratio device that can be quickly and reliably transitioned between different compression ratio modes of a variable compression ratio internal combustion engine.
The aforedescribed limitations and inadequacies of conventional internal combustion engines are substantially overcome by the present invention, in which a compression ratio apparatus is provided for selectively varying a compression ratio of an internal combustion engine. The apparatus includes a bearing retainer disposed between a connecting rod and a corresponding crankpin, the bearing retainer having an inner surface in communication with the crankpin and an outer surface in communication with the connecting rod, the connecting rod being axially movable relative to the bearing retainer along a longitudinal axis of the connecting rod to effect a selective displacement of the connecting rod relative to the bearing retainer, the displacement thereby causing a change in the effective length of the connecting rod and the compression ratio of the internal combustion engine. Preferably, at least one locking mechanism is provided in cooperation with the bearing retainer and the connecting rod for maintaining the connecting rod at a selected position relative to the bearing retainer, the selected position corresponding to a selected compression of the internal combustion engine.
A principal advantage of the above-described compression ratio apparatus is that transitions between two or more compression ratio modes of an internal combustion engine can be accomplished quickly and reliably without requiring the rotation of an eccentric ring member as disclosed by the prior art. Transitions can be completed within a single cycle of the internal combustion engine by allowing the compression ratio apparatus to respond to the inertial forces acting on the connecting rod and piston. Transitions can be further assisted and the connecting rods xe2x80x9clockedxe2x80x9d into position using a suitable hydraulic or electromechanical system. In a preferred embodiment, the engine""s oil system is used to actuate the mechanism to produce a selected compression ratio for the internal combustion engine.
In accordance with a related aspect of the present invention, a reciprocating internal combustion engine is also provided having the above-described compression ratio apparatus.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.